As illustrated by the above related patents, it is known to provide an electrical circuit incorporated in an identification card for identifying the bearer of the card. It is also known to provide means in the card circuit for sending a signal back to the receiver, which signal is individually encoded in a manner to identify the card holder from all other card holders. In such previous devices the circuit has either acted passively, that is, has resonated at a fixed frequency in response to a sweeping frequency, or in the alternative, has incorporated a battery and oscillator for generating a specific frequency not necessarily the same as that sent by the receiver. However, in examples where the card has included an oscillator, a power source has been provided to drive the oscillator, and there then exists the added technicalities of phase locking the receiver to the card-generated signal and providing a battery source within the card. The battery must have a useful life to give the card a sufficient longevity while not being overly large so as to require the card to be bulky.
An oscillator on the card has the difficulty of needing a crystal for stability, with consequent bulk and cost; or if no crystal then some means for frequency stability even with a fluctuating voltage power supply.
It is the purpose of the present invention to provide a card which generates a signal which is individually encoded to identify each particular card separate from the others and yet requires no built-in power source.
It is another purpose of the present invention to provide a card capable of generating an identification signal which is easily detectable by the reader and which rejects noise.
It is another purpose to provide a card needing no oscillator.